Abstract
Coastal protection has become a critical global priority due to escalating coastal erosion associated with sea-level rise and increasing storm surges. Conventional hard-engineering structures, while effective in reducing wave energy, often result in substantial degradation of marine ecosystems, raising concerns about long-term sustainability and ecological resilience. As a response, nature-based solutions (NBS), including oyster or coral reef restoration as part of living shoreline systems, have gained increasing attention for their potential to simultaneously provide coastal defense and enhance marine habitat quality. In addition to that submerged artificial reefs or breakwaters can be characterized as ecologically improved gray infrastructure or hybrid coastal defense systems that can promote secondary ecological functions by offering substrate for habitat development and species colonization, functioning similarly to nature-based solutions (NBS). Numerous investigations on both naturally occurring reefs and artificial reefs, conducted through laboratory experiments, numerical modeling, and field implementations assess the engineering performance of coastal protection services. This study aims to review current investigations systematically on naturally occurring reefs (oysters or coral) or submerged artificial reefs, evaluating their principal functions, limitations, and methodological approaches to attenuate wave energy while critically synthesizing the research approaches with corresponding findings to highlight the advancements in coastal protection services. The outcomes presented herein aim to guide coastal managers, engineers, researchers, and policymakers towards the development and implementation of sustainable, ecosystem-based coastal protection strategies that integrate both engineering efficacy and environmental enhancement.